1. Field of the Invention
The invention relates to a process for the coating of hollow bodies open on one side, such as the coating of metal cans or the like, wherein the individual hollow bodies are washed, coated inside and out, dried, optionally printed, again dried and subsequently beaded at the open end.
2. Background of the Art
The increasingly severe environmental protection regulations concerning lacquering processes make consideration of the electro-dipcoating process worthwhile for introduction in the can-making industry in the form of a fully automatic coating process. It is known to coat can bodies open on both sides for three-part cans or a weld seam electrophoretically by immersion in an electro-dipcoating bath. U.S. Pat. No. 3,694,336 and West German Offenlegungsschrift No. 2116715 are illustrative of this process. The bodies are easily handled in this known procedure, as they have no bottoms and the liquid of the bath can enter without hindrance and run off after the coating again without problems.
Hollow bodies closed on one end, such as cans with bottoms, cannot be coated electrophoretically in a simple manner as is necessary in order to obtain a uniform coating. The air in the hollow body must be allowed to escape completely. For this reason, the machine building industry has developed special methods to effect the process in steps. In other words, the coating is applied in individual, successive steps, for example, on the inside first. The means known for this purpose have certain common features. Thus, the cans are held at the bottom for the internal coating, while simultaneously establishing the necessary electrical contacts. A counter electrode is inserted in the can from the open side. This counter electrode must be located at a slight distance of 0.25 to 5 mm from the inner wall of the can. Thus, the shape of the electrode must be adapted very accurately to that of the can. In view of the complicated configuration of the corresponding apparatus, the cans must be coated individually in succession so that only very short coating times of 10 to 500 msec. are available if a high production rate is desired. In closed systems, for example in a vertical layout (European Pat. Nos. 50,045; 19,669; British Pat. No. 1,117,831; U.S. Pat. 3,922,213 and West German Offenlegungsschrift No. 2929570) the liquid must be pumped at high velocities in order to apply the electro-dipcoating liquid and a water rinse within short periods of time and to exhaust the gases formed during the electro-dipcoating process. Oxygen or hydrogen gases are formed depending on polarization. In open systems, the cans arranged in an approximately horizontal position must be rotated to obtain uniform coating as shown in West German Offenlegungsschrift No. 2633179 and U.S. Pat. No. 4,107,016. There is also a high risk of contamination in the blowing of the cans.
The disadvantage of these known configurations include the fact that the cans must be coated individually in succession, thereby requiring a great mechanical effort. The space needed for the apparatus almost renders an economic mass production impossible. Internal electrodes can be inserted with adequate fitting accuracy only in cans with straight and smooth walls, i.e., can shapes other than cylindrical lead to severe difficulties. Because of the slight distance of the internal electrode to the wall of the can, there is a danger of electrical shorts and flash-overs in zones of high current density. Correspondingly, coatings with a low layer resistivity must be used in order to be able to apply the coating with low electric voltages without interference in the short period of time available.